(a) Technical Field
The present invention relates to a system and method for recovering carbon dioxide from exhaust gas. More particularly, the present invention relates to a method for recovering carbon dioxide from exhaust gas for saving the cost for recovery of carbon dioxide by decreasing the energy required for recycling a carbon dioxide absorbent solution.
(b) Background Art
Korea is one of the chief greenhouse gas-emitting countries, which was ranked 6th in 2009 with 490 million tons of carbon dioxide emitted into the air. To cope with the global warming issue, the Korean government has implemented a plan to reduce greenhouse gas emissions by 30% by 2020 and has enacted the Framework Act on Low Carbon, Green Growth on Apr. 13, 2010.
Recognizing that the carbon capture and storage (CCS) technique will have a great impact on the reduction of greenhouse gas emission, the Korean government selected CCS as one of the 15 growth drivers and announced the National CCS Development & Promotion Plan in July 2010. According to this plan, carbon capture techniques will be developed and large-scale carbon dioxide (CO2) storage sites will be explored in stages. By 2017, carbon capture and storage will be linked to store more than 1 million tons of carbon dioxide annually. To this end, the government and the related industries will invest about 2 trillion won for technical development in this area by 2020.
The existing methods for capturing carbon dioxide include absorption, adsorption, membrane separation, etc. Among them, the absorption method is evaluated economical and easily applicable compared with the adsorption or membrane separation methods, since it allows processing of a large quantity of exhaust gas and provides good carbon dioxide removal efficiency even when the carbon dioxide concentration is as high as 7-30%.
Among the existing absorption methods, chemical absorption, which is being developed widely, allows selective separation of carbon dioxide from exhaust gas using chemical reactions. Accordingly, it is advantageous in that CO2 removal efficiency is high even when the partial pressure of CO2 is low since the absorption amount is not greatly affected by the partial pressure of CO2. However, it consumes a lot of energy during separation of the absorbent solution from CO2.
In general, energy cost accounts for over 60% of the CO2 recovery costs using the chemical absorption method. Of the energy cost, the cost spent for recycling of the CO2 absorbent solution accounts for at least 80%. Accordingly, in order to reduce the recovery cost of carbon dioxide and improve the carbon dioxide absorption technique, a technique allowing the reduction of energy required for the recycling of the absorbent solution is necessary.
As seen from FIG. 1, in the existing carbon dioxide absorption technique, an exhaust gas containing carbon dioxide is supplied to an absorption tower 1 packed with a filler having a large surface area so as to allow efficient gas-to-liquid contact and is in contacted with an absorbent in solution state sprayed from the upper portion of the absorption tower 1 under atmospheric pressure at around 40-70° C. in order to absorb the CO2 in the exhaust gas with the absorbent solution. The absorbent solution discharged from the absorption tower 1, i.e. the absorbent solution containing CO2, is heated to about 100-160° C. and recycled in a recycling tower 3, preheated by the absorbent solution discharged from the lower portion of the recycling tower 3 via heat exchange, and then supplied to the upper portion of the recycling tower 3. In the lower portion of the recycling tower 3, the absorbent solution containing CO2 is heated to 100-160° C. using a heater 4 such as a boiler to release CO2 and then discharged in the upper portion of the recycling tower 3. The vaporized absorbent solution which is discharged together with the released CO2 is cooled and condensed by a cooler 5, separated from carbon dioxide by a separation drum 6, refluxed to the recycling tower 3, and returned to the absorption tower 1 by being discharged in the lower portion of the recycling tower 3. The released, highly-concentrated (90-100%) CO2 in a gaseous state is separately recovered from the separation drum 6 for storage/fixation/conversion.
As the absorbent for such chemical absorption, amine-based absorbent solutions are commonly used. Examples of the amine-based absorbent solution include monoethanolamine, diethanolamine, triethanolamine, ethyldiethanolamine, diisopropylamine, diglycolamine, piperazine, 2-piperazineethanol, hydroxyethylpiperazine, 2-amino-methyl-1-propanol, 2-ethylamino-ethanol, 2-methylamino-ethanol, 2-diethylamino-ethanol, and others. They are commonly used as mixed with a solvent such as water in concentrations of 5-60 wt %.
As described above, in the existing technique for recovering carbon dioxide from exhaust gas, energy cost accounts for over 60% of the CO2 recovery cost, and the cost spent for recycling of the CO2 absorbent solution by heating in the lower portion of the recycling tower accounts for about 80% of the total energy cost. Also, a lot of energy is consumed to separate CO2 from the absorbent solution in order to recycle the absorbent solution, which further increases the cost for recovery of carbon dioxide.
That is to say, in the existing technique for recovering carbon dioxide, the absorbent solution containing that has absorbed CO2 is heated within the recycling tower 3 to separate CO2 and the absorbent solution is partly vaporized so as to elevate and maintain the pressure inside the recycling tower 3, so that the separated CO2 can be effectively discharged from the recycling tower 3. The vaporized absorbent solution discharged together with CO2 from the recycling tower 3 is cooled and condensed by the cooler 5, separated from CO2 in the separation drum 6, and returned to the upper portion of the recycling tower 3 in liquid state. Since a lot of energy is consumed when partly vaporizing the absorbent solution, the energy cost is increased and the cost for recovery of carbon dioxide is also increased.